1. Field of the Invention
The present invention relates in general to electrochromic devices, and more particularly, to an electrochromic medium for use in an electrochromic device which includes a current-reducing additive comprising a polymer present in an effective concentration to materially reduce the current of the electrochromic medium upon application of an electrical potential, relative to an electrochromic medium without the current-reducing additive, while maintaining a sufficiently low viscosity to facilitate acceptable production rates without a problematic degree of seal or other device component failure.
2. Background Art
Electrochromic devices have been known in the art for several years. While the utilization of electrochromic devices, such as electrochromic mirrors, has become increasingly popular, for example, among the automotive industry, the operating current of many electrochromic devices remains problematic for a plurality of reasons.
For example, one problem associated with electrochromic devices which, at least in part, can be attributed to current or electron flow is the formation of color bands proximate the electrodes of the electrochromic device, which is commonly referred to as “segregation.” Segregation can be especially prevalent after an electrochromic device is held in a low transmission or colored state for an extended period of time. Electrochromic windows are very susceptible to segregation inasmuch as, during normal operation, they may remain in a colored state for several, continuous hours.
A second problem associated with electrochromic devices that is directly related to current is that the electrochromic devices can generate a substantial amount of heat upon application of an electrical potential. The generated heat can, in turn, impart undesirable stress upon many of the electrical components used to control the electrochromic device. Often times, this requires many of the electrical components to be specially designed to handle extreme temperature variations for extended periods of time, which can materially increase the cost of the electrochromic device.
A third problem associated with electrochromic devices that is directly related to current is that the electrochromic devices can require a substantial draw of power during normal operation, which is a material concern with large area electrochromic devices, such as large exterior electrochromic mirrors for automobiles or electrochromic windows for residential, commercial, and/or industrial buildings.
One attempt to reduce current is disclosed in U.S. Pat. No. 5,142,407 (hereinafter the '407 patent). The '407 patent teaches incorporating cyanoethylsucrose (hereinafter “CES”), a non-polymer, into an electrochromic medium to reduce leakage current. However, the incorporation of CES into an electrochromic medium is undesirable for a plurality of reasons, including, but not limited to, the fact that CES can be costly, especially at the loading concentrations required to materially reduce leakage current. Moreover, the incorporation of an additive in such high concentrations may raise issues relative to the long term performance, stability, and/or operation of an associated electrochromic device.
It is therefore an object of the present invention to provide an electrochromic device which utilizes one or more relatively common and inexpensive low molecular weight polymers as current-reducing additives that remedy, or at least minimize, the aforementioned detriments and/or complications associated with the operating current of many presently available electrochromic devices while maintaining a sufficiently low viscosity to facilitate acceptable production rates without a problematic degree of seal or other device component failure.
In addition, if an electrochromic device is operating at an acceptable current, the incorporation of one or more additives of the present invention can enable an electrochromic device to be reconfigured with a smaller cell spacing while maintaining a generally constant current. Electrochromic devices having reduced cell spacing are beneficial for at least three reasons. First, electrochromic devices having reduced cell spacing can more easily achieve an acceptable image or viewing quality, especially for large-scale electrochromic devices. Second, electrochromic devices having reduced cell spacing contain less electrochromic medium, and, accordingly, have an inherently lower cost. They also weigh less, which can be important for small scale, mobile, and/or weight sensitive applications. Third, electrochromic devices having reduced cell spacing can be manufactured with increased efficiency.
These and other objectives of the invention will become apparent in light of the present specification, claims, and drawings.